Eric, Nice explanation. Good engineering practice. However, circuits can be loaded 100% as long as it is not a continuous load (operated for more than 4 hours). Vacuum cleaners (not likely to run 4 hours at a time) are more likely limited to 12 amps due to minimum circuit opacity for motor loads requiring an additional 25% of the load, so as to not overload a 15 amp circuit.
I apologize to all the non electricians reading this. Due to the amazingly confusing way the NEC is written, it's hard for electricians to pass up a good code argument. :) Once again, very good engineering practice, Eric, in spite of the code details. Gerald Pelnar WD0FYF McPherson, Ks ----- Original Message ----- From: "Eric Lemmon" <[EMAIL PROTECTED]> To: <Repeater-Builder@yahoogroups.com> Sent: Thursday, July 17, 2008 10:47 PM Subject: RE: RE: [Repeater-Builder] Re: RadioShack Recalls Power Supplies > Ron, > > That is a good question. The answer is that one is not supposed to > connect > any load greater than 12 amperes to an outlet rated at 15 amperes, that > is, > a NEMA 5-15R receptacle. The NEC allows two or more 15-ampere-rated > outlets > to be installed on a 20-ampere branch circuit (wired with 12 AWG > conductors > and a 20A fuse or circuit breaker), but the limit of 12 amperes on each > outlet still applies. > > Proof of this restriction is evident in the "vacuum cleaner wars" of a > decade or so ago. Hoover came out with a vacuum cleaner with "7 amperes > of > cleaning power." Then Bissel came out with a unit claiming 9 amperes of > cleaning power. Other vacuum cleaner makers entered the fray until all of > the brands had units with "12 amperes of cleaning power." The reason that > nobody offered a unit with 13 amperes of cleaning power is because they > would then have to equip that unit with a NEMA 5-20P plug and at least a > 14/3 power cord. Most older homes do not have NEMA 5-20R outlets, so such > a > vacuum cleaner could not be plugged in to the outlets in most homes. > Besides, there is no credibility to a laughable rating of "cleaning power" > expressed in amperes! That is about as silly as claiming that a mobile > radio has "13.8 volts of talk power!" > > Back to your second question. By definition, a 15-ampere-rated branch > circuit has circuit conductors of #14 AWG or larger, and is protected by a > fuse or circuit breaker rated at 15 amperes. The fuse or circuit breaker > should hold indefinitely at 15 amperes, but the NEC recognizes that > allowing > 100% of rated current is never a good idea, since wiring in attics may > already be in a very hot environment. Therefore, the NEC requires that no > ordinary branch circuit be permitted to be loaded more than 80% of the > circuit rating. That's where the 12 and 16 ampere limits come from. > > Another issue is voltage drop, which is directly proportional to circuit > loading. Circuits that are loaded to 100% of rating will probably have > excessive voltage drop, which leads to inefficient operation. Good > electrical design dictates that the wire size be increased for long runs, > to > keep the voltage drop below 3%. Moreover, an adequate electrical supply > system should never experience more than 80% loading of any circuit. Very > heavy single loads should have a dedicated branch circuit of suitable > capacity, with a single outlet. > > 73, Eric Lemmon WB6FLY > >
